1. Field of the Invention
The present invention generally relates to electrical signal transmission systems and, more particularly, is concerned with a signal transmission system incorporating parallel planar transmission lines for differential-mode signal transmission.
2. Description of the Prior Art
Shielded twisted pair transmission lines are often used to carry electrical signals such as digital pulses with fast rise/fall times and high-speed clock frequencies between a digital system under test and the test equipment evaluating the performance of that system. Overall, the shielded twisted pair transmission lines provide a low loss signal transmission system that delivers the signals with good fidelity, contributing minimal effects to the signal shape.
In particular, heretofore, shielded twisted pair transmission lines have been the preferred transmission system for differential-mode signal transmission applications. The differential signal mode is used to minimize signal losses, and interference from noise signals on transmission lines. These are necessary properties for any transmission line carrying low level signal amplitudes. Single-ended transmission line modes used on coaxial, stripline and microstrip transmission lines can pick up considerable levels of noise signals and common mode currents. The differential signal transmission mode minimizes these signal interference problems.
Although shielded pair transmission lines have been the preferred transmission system for their capability of transmitting in the differential signal mode, they create problems in connecting their low amplitude signals to circuitboards. Particularly, the problems arise when they must interface with different types of transmission line systems such as coaxial cable and stripline. If the differential-mode, shielded twisted pair transmission lines are connected to controlled impedance, single end lines such as coaxial cable, stripline or microstrip, then the above-described advantages of signal transmission in the differential mode are lost. While a controlled characteristic impedance is maintained on these three types of single ended lines, the initial differential-mode signal will suffer increased signal losses and will pick up noise signal interference.
If differential-mode, shielded twisted pair transmission lines are connected to parallel printed circuitboard traces, the differential-mode will be preserved, but now a characteristic impedance mismatch will occur. Signal losses from reflections and losses in signal fidelity will result.
Therefore, a need exists for an alternative signal transmission system to avoid the aforementioned problems in interfacing shield twisted pair transmission lines with circuitboards.